The invention relates to an axial piston compressor, especially a compressor for the air-conditioning system of a motor vehicle, having a housing and, for drawing in and compressing a coolant, a compressor unit arranged in the housing and driven by means of a drive shaft, the compressor unit comprising pistons, which move axially back and forth in a cylinder block, and a tilt plate, for example in the form of tilt ring, wobble plate or swash plate, which drives the pistons and rotates together with the drive shaft.
An axial piston compressor of such a kind is known, for example, from DE 197 49 727 A1. That compressor comprises a housing in which, in a circular arrangement, a plurality of axial pistons are arranged around a rotating drive shaft. The drive force is transmitted from the drive shaft, by way of a member for conjoint movement, to an annular tilt plate and in turn, from there, to the pistons displaceable in translation parallel to the drive shaft. The annular tilt plate is pivotally mounted on a sleeve which is mounted on the drive shaft so as to be axially displaceable. In the sleeve there is provided an elongate hole, through which the mentioned member for conjoint movement engages. Consequently, the capability of the sleeve for axial movement on the drive shaft is limited by the dimensions of the elongate hole. Assembly is carried out by passing the member for conjoint movement through the elongate hole. The drive shaft, member for conjoint movement, sliding sleeve and tilt plate are arranged in a so-called drive mechanism chamber, in which gaseous working medium of the compressor is present at a particular pressure. The delivery volume and consequently the delivery output of the compressor are dependent on the pressure ratio between the suction side and delivery side of the pistons or correspondingly dependent on the pressures in the cylinders on the one hand and in the drive mechanism chamber on the other hand.
A somewhat different kind of construction of an axial piston compressor is described, for example, in DE 198 39 914 A1. The tilt plate is in the form of a wobble plate, there being arranged between the wobble plate and the pistons a non-rotating take-up plate mounted opposite the wobble plate.
The compressor shown in FIG. 13 is known from EP 1 172 557 A2. That compressor has a tilt plate arrangement having a tilt plate in the form of a swash plate 118, with which the pistons 120 are in articulated connection by way of sliding blocks 121. The tilt plate arrangement also has a supporting arrangement which simultaneously, in the form of a part for conjoint movement, transmits torque between a drive shaft 114 and the swash plate 118.
A first component for conjoint movement 117, which is fixed to the drive shaft 114 and is arranged as a bearing, in the form of a receiving bore, for the part for conjoint movement is disposed a substantial distance to the side of the swash plate 118, and a second component for conjoint movement 119, which engages in the first in a manner allowing articulation, is in the form of a lateral extension to the swash plate 118. The above-described structure of the swash plate in the form of the components for conjoint movement 117 and 119, which are present as pairs, causes the swash plate arrangement to have an outwardly displaced centre of gravity. The centre of gravity's being remote from the tilt axis and also from the tilt-providing articulation gives rise to an imbalance because the drive mechanism can be balanced only for a preferably medium tilt angle of the swash plate. It is to be noted that the centre of gravity moves in dependence on the tilt angle at a considerable distance from the tilt-providing articulation, which represents the centre of the tilt movement. The swash plate 118 furthermore has a thickened hub portion and has, as explained above, a relatively large moment of inertia due to the components for conjoint movement 117 and 119 together with a centre of gravity which is far removed from the tilt axis so that a sudden change in the speed of rotation results in adjustment of the inclination of the swash plate 118 with corresponding inertia.
The location of the centre of gravity also plays an important part in governing regulation behaviour. The regulation behaviour is influenced in such a way that the compressor undergoes a high degree of up-regulation, that is to say the mass forces of inertia of the swash plate and the location of its centre of gravity give rise to a moment of deviation J, which in turn generates a tilt moment MSW=J×ω2. The afore-mentioned tilt moment always opposes the moment of deviation J. This generally means, in the case of compressors according to the prior art, a reduction in tilt angle, especially in the working range at medium and larger tilt angles. Of course, various moments of deviation generally act on a component, the moment of deviation mentioned here being the moment of deviation that is relevant to the tilt movement of the swash plate. This moment of deviation is caused by the sole degree of freedom present in the system, which degree of freedom is due to the tilt-providing articulation.
An arrangement like that described above is put into practice, for example, in the mass-produced compressor 6SEU 12 C of DENSO, in which R134a is used as coolant. The (relevant) moment of deviation J of the swash plate gives rise to a tilt moment MSW about the centre of the tilt movement of the swash plate which has an effect, at least in the region of medium and larger swash plate tilt angles, such that the tilt angle of the swash plate tries to decrease. The mass forces of the pistons give rise (by way of their excursion) to a tilt moment Mk,ges at the swash plate which is likewise applied about the centre of the tilt movement of the swash plate. In contrast to the tilt moment MSW of the swash plate, the tilt moment produced by the pistons has an effect in the direction of an increase in the tilt angle of the swash plate. The mass centre of gravity of the system, which is located outside the tilt point or point of rotation of the swash plate, reinforces the effect of the pistons. The effect brought about by the centre of gravity is generally a contributing factor in calculation of the (total) moment of deviation, where it is taken into account by means of a so-called Steiner component.
In relation to the mentioned 6SEU 12 C compressor of DENSO, it is to be noted that the mass of a tilt plate cannot be increased at will in order to modify the regulation behaviour accordingly. This is due to the fact that, in the case of the compressors of the described kind, the mass centre of gravity of the tilt plate is generally a substantial distance away from the tilt-providing articulation of the tilt plate. The basic justification for such an arrangement is that the tilt plate, in addition to its own guideway on the drive shaft, has to be coupled, by way of a positioning mechanism, to the drive shaft or a component connected to the drive shaft (component for conjoint movement).
The mentioned distance between the centre of gravity of the tilt plate and the tilt-providing articulation thereof results in imbalance of the drive mechanism, especially in dependence upon the tilt angle of the tilt plate (the centre of gravity moves “in the manner of a swing” beneath the tilt-providing articulation), and in the worst case results in an up-regulating characteristic (so-called “location of centre of gravity”).
Future compressors should not have, in the region of the tilt plate, an outwardly displaced location of centre of gravity, and the imbalance due to the drive mechanism, which is brought about especially by the tilt plate, should be small or, ideally, zero.
Generally it is the following moments which in the centre of the tilt movement of the tilt plate have an influence on the tilting of the tilt plate. The direction of the moment is given in brackets, with (−) denoting down-regulation (in the direction of minimum stroke) and (+) denoting up-regulation (in the direction of maximum stroke):                moment due to gas forces in the cylinder spaces (+)        moment due to gas forces from the drive mechanism chamber (−)        moment due to a restoring spring (−)        moment due to an advancing spring (+)        moment due to rotating masses (−); including moment due to location of centre of gravity (for example, tilt plate: tilt location≠mass centre of gravity): can be (+) or (−)        moment due to masses moved in translation (+)        
In the case of changes in the speed of rotation and, at the same time, substantially constant operating conditions, it is only the last two mentioned moments, namely the moment due to rotating masses and the moment due to masses moved in translation, which affect the regulation behaviour. In this context what is decisive is, especially, the net result of the forces and moments about the centre of the tilt movement of the tilt plate.
In the case of compressors of modern design, it is desirable to provide an up-regulating moment in the region of small tilt angles of the tilt plate, whereas at medium and larger tilt angles a markedly down-regulating moment is favoured for the tilt plate.
In this context, reference is to be made to EP 0 809 027, in which it is pointed out that it is desirable in the case of compressors to provide constant regulation of the delivery quantity. In the afore-mentioned publication it is proposed that the kinematics of a compressor be so designed that the down-regulating tilting moments acting on the tilt plate of the compressor should clearly predominate over the up-regulating tilting moments.
In this context it should be mentioned that the phrase “delivery quantity” is relatively imprecise. The delivery quantity could be considered constant if, for example, on doubling the speed of rotation, the tilt angle of the tilt plate halves. As a result the delivery quantity would be constant in geometric terms. Of course, other parameters will also then have an effect on the delivery quantity when the tilt angle of the tilt plate changes, for example volumetric efficiency, oil throw-off or the like.
For constant regulation of the delivery quantity in the event of changing speeds of rotation, the restoring torque of the tilt plate is utilised because the tilt plate opposes its angled position because of the dynamic forces at the co-rotating plate part. This process can be aided by the force of a spring so that the increasing quantity delivered in the case of an increase in the speed of rotation is at least partly compensated by restoration of the angled or pivoted position of the tilt plate.
For a better understanding, the described tilting behaviour due to variation in the speed of rotation is shown in FIGS. 1 and 2. FIG. 1 shows the dependence of drive mechanism chamber pressure difference, relative to the suction pressure, set against the tilt angle α, or “alpha”, of the tilt plate. Calculations were carried out by way of example for the following pressures:
high pressure 120 bar and suction pressure 35 bar.
Also calculations were carried out using the speeds of rotation:
600 rpm, 1200 rpm, 2500 rpm, 5000 rpm, 8000 rpm and 11,000 rpm.
In FIG. 1, however, only five of the six plots calculated are to be seen. This is due to the fact that the plots for the speeds of rotation 600 rpm and 1200 rpm lie substantially entirely on top of one another (because of a lacking dynamic); accordingly the “delivered quantity that is independent of the speed of rotation”, which is required in the prior art, is rather a wishful notion that cannot be put into practice using the measures described.
Referring to the diagram according to FIG. 1, it can be very clearly seen that plots are obtained which cause the tilt plate to adopt greater tilt angles when the speed of rotation increases. In this context it should be mentioned that FIG. 1 is to be regarded only as an example using a simple geometry. However, the trend shown also applies to more complex geometries. The calculation was based on a tilt ring having a predetermined internal and external diameter and a predetermined height.
Also of relevance are the piston mass, the reference diameter on which the pistons are located, and the number of pistons.
The tilt ring preferably has a mass moment of inertia J2=Jη or J=m/4 (ra2ri2+h2/3) which is greater than 100,000 gmm2. Preferably, the mass moment of inertia is greater than J=200,000-250,000 gmm2.
Furthermore, the tilt ring preferably has a mass moment of inertia of
      J    3    =            J      ζ        =                  m        2            ⁢              (                              r            a            2                    +                      r            i            2                          )            which is greater than 200,000 gmm2, preferably about 400,000-500,000 gmm2.
There is described hereinbelow the derivation of the so-called moment of deviation, which governs the tilting of the tilt plate or tilt ring and which, more particularly, in the case shown is solely responsible for the tilting of the tilt plate or tilt ring provided that the mass centre of gravity of the tilt plate or tilt ring is located both at the tilting point and also at the geometric centre-point of the tilt plate or tilt ring. This represents an ideal case of the arrangement that is to be aspired to. For the derivation of the moment of deviation the following very generally applies, with reference to FIG. 3:Jyz=−J1 cos α2 cos α3−J2 cos β2 cos β3−J3 cos γ2 cos γ3 α1=0 Direction angles of the x axisβ1=90° relative to the main inertia axes ξ·η·ζγ1=90°α2=90°β2=ψDirection angles of the y axisγ2=90°+ψ relative to the main inertia axes ξ·η·ζα3=90° Direction angles of the z axisβ3=90°−ψ relative to the main inertia axes ξ·η·ζγ3ψ
            J      2        =                  J        η            =                        m          4                ⁢                  (                                    r              a              2                        +                          r              i              2                        +                                          h                2                            3                                )                                J      3        =                  J        ζ            =                        m          2                ⁢                  (                                    r              a              2                        +                          r              i              2                                )                    (Note: J3≈2 J2 Aim: Jyz should have a particular magnitudeJyz↑J3↑J2 necessarily increases!)Moment of DeviationJyz=−Jz cos ψ sin ψ+J3 cos ψ sin ψ
The following holds true independently of FIG. 3:
Moment Due to Mass Force of the Pistons
      β    i    =      θ    +          2      ⁢              π        ⁡                  (                      i            -            1                    )                    ⁢              1        n            Zi=R·ω2 tan α cos βi Fmi=mk·zi M(Fmi)=mk·R·cos βi·zi
      M          k      ,      ges        =                    m        k            ·      R        ⁢                  ∑                  i          =          1                n            ⁢                                    z            i                    ·          cos                ⁢                                  ⁢                  β          i                    Moment Msw Due to Moment of DeviationMSW=Jyz·ωz
            J      yz        =                  {                                                            m                ⁢                                                                  ⁢                s                ⁢                                                                  ⁢                w                            2                        ⁢                          (                                                r                  a                  2                                +                                  r                  i                  2                                            )                                -                                                    m                ⁢                                                                  ⁢                s                ⁢                                                                  ⁢                w                            4                        ⁢                          (                                                r                  2                  2                                +                                  r                  i                  2                                +                                                      h                    2                                    3                                            )                                      }            ⁢      cos      ⁢                          ⁢      αsin      ⁢                          ⁢      α                  J      yz        =                            m          ⁢                                          ⁢          s          ⁢                                          ⁢          w                24            ⁢      sin      ⁢                          ⁢      2      ⁢              α        ⁡                  (                                    3              ⁢                              r                a                2                                      +                          3              ⁢                              r                i                2                                      -                          h              2                                )                    MSW≧Mk,ges                 or        
  [                    ω        2            ⁢                        R          2                ·                  m          k                    ⁢      tan      ⁢                          ⁢      α      ⁢                        ∑                      i            =            1                    n                ⁢                              cos            2                    ⁢          β                      ≦                  ω        2            ⁢                        m          ⁢                                          ⁢          s          ⁢                                          ⁢          w                24            ⁢      sin      ⁢                          ⁢      2      ⁢                          ⁢              α        ⁡                  (                                    3              ⁢                              r                a                2                                      +                          3              ⁢                              r                i                2                                      -                          h              2                                )                      ]
The variables used above have the following meanings:    θ rotation angle of the shaft (the considerations above and below being made on the basis of θ=0 for the sake of simplicity)    η number of pistons    R distance from piston axis to shaft axis    ω speed of rotation of shaft    α tilt angle of tilt ring/tilt plate    mk mass of a piston including sliding blocks or pair of sliding blocks    mk,ges mass of all pistons including sliding blocks    msw mass of tilt ring    ra external radius of tilt ring    ri internal radius of tilt ring    h height of tilt ring    ρ density of tilt ring    V volume of tilt ring    βi angle position of piston i    zi acceleration of piston i    Fmi mass force of piston i (including a pair of sliding blocks)    M(Fmi) moment due to mass force of piston i    Mk,ges moment due to mass force of all pistons    Msw moment due to advancing moment of tilt ring/tilt plate or due to moment of deviation (Jyz)    J=f(ρ, r, h) mass moment of inertia
Specifically, FIG. 1 was based on the following tilt plate or swash plate tilt moment determination, wherein α was varied from 0° to 16°:
Determination of swash plate tilt momenttheta  00.00[*]n (p)  7—betaiJz208436R  29[mm]beta10.00.00n 2500[1/min]beta251.40.90(jx =) Jy106137alpha  160.28[*]beta3102.91.80mk  45[g]beta4154.32.69Jyz27105mk,ges 315[g]beta5205.73.59beta6257.14.49omega262msw 230[g]beta7308.65.39ra  37[mm]ri  21[mm] h  10[mm]z″i z″1569.9 rho  7.9[g/cm3]z″2355.4 z″3−126.8 z″4−513.5 z″5−513.5 z″6−126.8 V29154[mm3]z″7355.4msw/mk,ges0.73FmiiR fr,eing  30Fmi125.6R f(ra:ri)  29Fmi216.0Fmi3−5.7Fmi4−23.1Fmi5−23.1sin2(alpha  0.5299Fmi6−5.7tan(alpha)   0.2867Fmi716.0M(Fmi)iM(Fmi)10.74M(Fmi)20.29M(Fmi)30.04M(Fmi)40.60M(Fmi)50.60M(Fmi)60.04M(Fmi)70.29 n alpha 2500   16[1/min] [*]
It can be seen that the influence of the piston masses predominates, resulting in up-regulation behaviour of the swash plate or tilt plate with increasing speed of rotation. In this case, therefore Mk,ges>MSW.
FIG. 2 shows a diagram for an almost identical drive mechanism, which diagram results from the following calculation scheme, a being varied from 0° to 16° in this case too:
Determination of swash plate tilt momenttheta  00.00[*]n(p)  7—betaiJz375185R  29[mm]beta10.00.0n 2500[1/min]beta251.40.90(Jx =) Jy198786alpha  160.28[*]beta3102.91.80mk  45[g]beta4154.32.69Jyz46739mk,ges 315[g]beta5205.73.59beta6257.14.49omega262msw 415[g]beta7308.65.39ra  37[mm]ri  21[mm] h  18[mm]z″i z″1569.9 rho  7.9[g/cm3]z″2355.4 z″3−126.8 z″4−513.5 z″6−126.8 V52477[mm3]z″7355.4msw/mk,ges1.32FmiiR fr,eing  30Fmi125.6R f(ra:ri)  29Fmi216.0Fmi3−5.7Fmi4−23.1Fmi5−23.1sin2(alpha  0.5299Fmi6−5.7tan(alpha)  0.2867Fmi716.0M(Fmi)iM(Fmi)10.74M(Fmi)20.29M(Fmi)30.04M(Fmi)40.60M(Fmi)50.60M(Fmi)60.04M(Fmi)70.29 n alpha 2500    16[1/min] [*]
In this case, Mk,ges<MSW.
This calculation scheme shows that, compared to the calculation relating to FIG. 1, the thickness or height of the swash plate or tilt plate was increased from 10 mm (FIG. 1) to 18 mm (FIG. 2). The consequence thereof is that the relevant mass moment of inertia Jz is increased to about twice its value in comparison. In FIG. 2, there can be seen down-regulation behaviour of the tilt plate drive mechanism. This tendency is indicated by the arrow “n” in FIG. 2, “n” meaning the speed of rotation of the tilt plate and drive shaft. The arrow “n” in FIG. 1 has, of course, the same meaning, but in that case the arrow points in the opposite direction, which is intended to indicate up-regulation with increasing speed of rotation.
FIG. 1 reflects the prior art. In that context, the up-regulation behaviour corresponding to FIG. 1 is frequently to be found in current mass-produced R134a compressors. In the case of more recent developments, on the other hand, attempts are being made to change this tendency to the opposite, namely to that corresponding to FIG. 2.
In FIG. 4 there is also shown the case where the down-regulating tilt moments due to the mass moments of inertia/moments of deviation of the tilt plate or tilt plate component group are so dimensioned that there results a regulation behaviour in which, in the event of an increase in the speed of rotation, the tilt angle of the tilt plate remains almost constant, or decreases, in which case, as a result, at least part of the increasing delivery output resulting from the increase in the speed of rotation is compensated.
The graph in FIG. 4 is based on the following calculation:
Determination of swash plate tilt momenttheta  00.00[*]n (p)  7—betaiJz297897R  29[mm]beta10.00.00n 2500[1/min]beta221.40.90(Jx =) Jy154552alpha  160.28[*]beta3102.91.80mk  45[g]beta4154.32.69Jyz37981mk,ges 315[g]beta5205.73.59beta6257.14.49omega262msw 329[g]beta7308.65.39ra  37[mm]ri  21[mm] h  14.292[mm]z″i z″1569.9 rho  7.9[g/cm3]z″2355.4 z″3−126.8 z″4−513.5 z″5−513.5 z″6−126.8 V41667[mm3]z″7355.4msw/mk,ges1.04FmiiR fr,eing  30Fmi125.6R f(ra;ri)  29Fmi216.0Fmi3−5.7Fmi4−23.1Fmi5−23.1sin2(alpha  0.5299Fmi6−5.7tan(alpha)  0.2867Fmi716.0M(Fmi)iM(Fmi)10.74M(Fmi)20.29M(Fmi)30.04M(Fmi)40.60M(Fmi)50.60M(Fmi)60.04M(Fmi)70.29 n alpha 2500    16[1/min] [*] n alpha 2500   1[1/min] [*] n alpha 2500   8[1/min] [*] n alpha 2500   16[1/min] [*] n alpha 1100   1[1/min] [*] n alpha11000   8[1/min] [*] n alpha11000   16[1/min] [*]
FIGS. 5, 6 and 7 show the tilt moments MSW, Mk,ges corresponding to FIGS. 1, 2 and 4 and also the sums of the two afore-mentioned moments for a speed of rotation in dependence on the tilt angle of the tilt plate or geometric stroke displacement volume of the compressor. In FIG. 5 the up-regulating characteristic of the compressor can be clearly seen by virtue of the sum of the moments being in the positive region, whereas in FIG. 6 the sum of the moments is negative for all tilt angles of the swash plate. A compressor which follows the plot of the moments according to FIG. 6 has a down-regulating characteristic. Lastly, reference may be made to FIG. 7, which shows a moment plot where the moments Mk,ges and MSW are approximately equal so that the sum of moments for all tilt angles of the swash plate is approximately zero.
FIG. 8a gives the sum of MSW+Mk,ges for various speeds of rotation. FIG. 8a corresponds to FIGS. 1 and 5 and clearly shows an up-regulating total moment which Increases as the speed of rotation increases.
FIG. 8b shows the afore-mentioned sum for the case dealt with in FIGS. 2 and 6; it can be clearly seen that an increasingly down-regulating moment is obtained as the speed of rotation increases.
It should be pointed out that, in the case of FIGS. 8a and 8b, the moment of deviation of the tilt plate is equal to zero at the tilt angle 0°. This means that, in this example, the tilt-providing articulation and the mass centre of gravity of the swash plate coincide, in which case there is no outwardly displaced location of centre of gravity.
The diagram in FIG. 8c shows the behaviour of a compressor in which the moment of deviation and the resulting tilt moment MSW+Mk,ges at a swash plate tilt angle of 0° is not equal to zero. This results in start-up of the compressor being assisted by a moment; however, the following problems occur: As a result of the fact that at a very small tilt angle of, for example, 0° MSW Mk,ges is already not equal to zero, the said amount continues to be present throughout the tilt angle range. The plot is accordingly shifted approximately parallel to a plot having a starting value where MSW+Mk,ges is equal to zero. It also has to be taken into account that, in the region of relatively large tilt angles, the up-regulating effect is increased by the additional tilt moment (cf. FIG. 8a in this respect). As the trend in the case of modern compressors is towards regulation behaviour which rather follows the plots of FIGS. 2 and 6 and/or 4 and 7, up-regulating behaviour in the region of relatively large tilt angles is undesirable.